As refrigerants for a cold storage chamber, a car air-conditioner, a room air-conditioner, an industrial refrigerator and the like, 1,1,1,2-tetrafluoroethane (R134a) of hydrofluorocarbon (HFC), R410A that is a mixed solvent of difluoroethane (R32) and pentafluoroethane (R125) in a mass ratio of 1/1, and the like are conventionally widely used.
The global warming potential (GWP) of such a HFC refrigerant is, however, as high as 1000 or more, and therefore, the use is now becoming restricted by the so-called F-gas regulation. It is indispensable for an alternative candidate for the HFC refrigerant to have a low GWP, and one of the candidates is a hydrocarbon refrigerant such as isobutane (R600a) already practically used for a cold storage chamber. Since the hydrocarbon refrigerant has an extremely low GWP of 20 or less and suitable physical properties, extensive studies are being made on it.
In the coexistence of a hydrocarbon refrigerant, since a hydrocarbon molecule does not contain chlorine or fluorine for increasing lubricity, lubricity improvement by the refrigerant cannot be expected differently from a case of the HFC refrigerant or the like, and in addition, since solubility of a hydrocarbon in a refrigerating machine oil is so high that the viscosity of the oil is lowered, a lubrication condition becomes severe, which requires a higher wear resistance of a refrigerating machine oil than in a conventional technique. Besides, from the viewpoint of power saving, it is required to also have a performance capable of retaining a friction coefficient low.
In the field of lubricating oils widely, an oiliness agent such as an alcohol, an ester or a long-chain fatty acid, an anti-wear agent such as a phosphoric acid ester or a metal dithiophosphate, and an extreme pressure agent such as an organic sulfur compound or an organic halogen compound are generally known as anti-wear additives. On the other hand, in a refrigerating machine oil, an additive cannot be used unless it is precipitated in the coexistence of a refrigerant and harmfully affects stability. Therefore, in a conventional refrigerating machine oil, an alcohol-based or ester-based oiliness agent, or triphenyl phosphate or tricresyl phosphate among phosphates is used.
Beside, Patent Literature 1 discloses a lubricating oil used for a refrigerating machine containing a phosphorus-based additive and a specific epoxy compound together, Patent Literature 2 discloses a compressor lubricating oil, to be used with the HFC refrigerant, containing triphenyl phosphate and tri(alkylphenyl) phosphate together, and Patent Literature 3 discloses a refrigerating machine oil, to be used with the ITC refrigerant, containing tricresyl phosphate and glycidyl ether epoxy or carbodiimide.